This Invention relates to the treatment of coal ash to remove ammonia compounds that contaminate the ash as part of post-combustion treatments of exhaust gases to remove nitrogen compounds.
In 1990, the United States Environmental Protection Agency put into place the Clean Air Act Amendments which were designed to reduce the emissions of xe2x80x9cgreenhouse gasesxe2x80x9d. Among the emissions covered are the nitrogen compounds NO and NO2 referred to generically as NOx. NOx is generated through the combustion of coal and its generation is directly affected by combustion temperature, residency time and available oxygen. Several technologies have been developed to meet the mandated NOx reduction limits.
The NOx reduction technologies fall into two major categories. One category includes technologies that modify or control the combustion or firing characteristics. The effect of these approaches has been an increase in residual unburned carbon in the coal ash. The other category includes technologies that are employed after combustion has taken place. These technologies include selective non-catalytic reduction, selective catalytic reduction, and amine enhanced fuel lean gas reburn. These technologies involve adding ammonia (NH3), and a significant amount of the ammonia finds its way onto the coal ash, typically by combining with available sulphur and other compounds that attach to the ash particles.
Coal ash is a marketable product if it is not contaminated. The ash may be used, for example, in concrete products as a replacement for a portion of the cement. However, ash that has been treated to reduce NOx and which is contaminated either by unburned carbon or ammonia compounds is not marketable.
Systems have been developed which may be used to reduce the amount of ammonia compounds affixed to fly ash. For example, German Patent Application No. 3526756 describes a process for the reduction of ammonia residues from the treated fly ash of a steam generator. In this process, ammonia contaminated fly ash is treated with a hot treating gas, e.g., air heated to 700xc2x0 C. to 800xc2x0 C., so that the ammonia Is driven off into the treating gas. In U.S. Pat. No. 4,911,900, there is disclosed an apparatus in which fly ash containing ammonium salts is recovered from the flue gas of a combustion chamber and then introduced Into a fluidized bed apparatus in which ammonia is driven off from the fly ash. The stripping of ammonia from the fly ash is affected in the fluidized bed apparatus by direct heating with a hot gas at 700xc2x0 C. to 800xc2x0 C. Ammonia containing gas from the fluidized bed apparatus is then supplied to an ammonia scrubber from which an ammonia solution is removed for storage. While these systems may be suitable for certain installations, they do have efficiency problems, such as excessive heat loss, that may limit their use in cost sensitive installations.
Therefore, there is a need for an improved method and apparatus that can solve the unburned carbon and ammonia contamination problems in fly ash. More particularly, there is a need for an improved method and apparatus that can remove ammonia compounds from fly ash, and/or consume unburned carbon in fly ash to make the fly ash a marketable product.
Accordingly, the present invention is directed to a method and apparatus for the application of heat to remove ammonia compounds from fly ash, thereby making the fly ash a marketable product. The method of the invention is directed to a method for reducing the amount of ammonia compounds affixed to fly ash including the steps of providing an amount of fly ash wherein at least a portion of the amount of fly ash comprises particulates having ammonia compounds affixed to the particulates, and exposing the fly ash to flowing air having a temperature of at least 1,500xc2x0 F. (815xc2x0 C.) such that the fly ash is maintained in the flowing air until the fly ash reaches a temperature of at least 900xc2x0 F. (482xc2x0 C.).
The apparatus of the invention is directed to an apparatus for reducing the amount of ammonia compounds affixed to fly ash wherein the apparatus includes a source of fly ash comprising particulates having ammonia compounds affixed to at least some of the fly ash particulates, a heating chamber including a treatment bed comprising a media having openings, a fly ash supply conduit in communication with the heating chamber and the source of fly ash for transferring fly ash from the source of fly ash to the treatment bed of the heating chamber, a source of heated air, an air supply conduit in communication with the source of heated air and the heating chamber for providing a flow of heated air to the treatment bed of the heating chamber for contacting the fly ash on the treatment bed with the flow of heated air, a heated air conduit in communication with the heating chamber for transferring the flow of heated air from the heating chamber, and an ash removal conduit in communication with the heating chamber for transferring heated fly ash from the heating chamber.
Fly ash is a particulate material collected from the effluent, or flue gases, of stationary combustion sources, such as coal, oil, or municipal solid waste burning power plants. It generally comprises a very fine dust with particles mostly in the silt size range. The physical and chemical properties of fly ash vary according to the combustion source, depending on the source of fuel, burning and handling methods, and also the addition of materials both prior to and subsequent to combustion (such as nitrogenous NOx reducing treatment agents), as well as the addition of materials to aid in the fly ash collection process.
The principle constituents of fly ash are silica (silicon dioxide, SiO2), alumina (aluminum oxide, Al2O3), calcium oxide (CaO), and iron oxide (FeO). As is well known, the actual composition of fly ash can vary widely, and in fact, the ranges provided are merely representative of common fly ash compositions. In addition to those compositions detailed above, common combustion fly ashes also contain other components, as well as unburned carbon. Generally, fly ashes have a specific gravity which can range between about 2.1 and about 2.6. Most of the particles of fly ash are glassy spheres, except for the carbon particles, which are somewhat larger and more angular than the inorganic particles. Fly ash particles generally have an average size which can range from less than about 1.0 to about 80 microns in diameter, more commonly between about 1.0 and about 30.0 microns in diameter.
The fly ash is generated by the combustion of coal, oil, municipal solid waste, or other materials in a stationary combustion source such as an electricity generating utility or other like system. The fly ash is carried along with the combustion effluent, which often contains nitrogen oxides, a known pollutant. Nitrogen oxides are created when atomic oxygen and nitrogen are formed in the high temperature atmosphere generated.
The nitrogen oxides level in the combustion effluent can be reduced by the introduction of a nitrogenous treatment agent, either in and of itself or to facilitate a selective catalytic reduction process. The most common agents for the non-catalytic reduction of nitrogen oxides are urea and ammonia, which can be introduced according to a number of different processes effective for reducing nitrogen oxides. The catalytic reduction of nitrogen oxides generally involves passing the effluent across the catalyst bed in the presence of ammonia. Selective catalytic reduction processes for reducing NOx are well known and utilize a variety of catalytic agents.
Regardless of the specific method by which nitrogen oxides are being reduced by the introduction of a nitrogenous treatment agent, it is virtually impossible to prevent at least some ammonia from affixing to fly ash. When this occurs, ammonia becomes affixed to the fly ash, primarily by being reacted therewith, and is absorbed onto the fly ash. Most often, the ammonia is present on the ash in the form of ammonium salts, such as ammonium chloride, ammonium sulfate, and ammonium bisulfate, as well as absorbed free ammonia. As used herein, the term xe2x80x9cammonia compoundsxe2x80x9d shall be used to refer to these substances, that is, ammonia (NH3) and compounds including the ammonium ion (NH4).
In order to prevent the fly ash from being emitted to the atmosphere, it is collected by conventional apparatus such as in a baghouse, an electrostatic precipitator, or like means. The ash is then transported either to disposal, such as in a landfill or strip mine, or to be r used such as for structural fill or In the formation of cement or other building materials.
In order to liberate or strip the affixed ammonia from the combustion fly ash after it has been collected as described above, it is exposed to moving air under conditions effective to strip a substantial amount of the ammonia from the ash. Although this description is written in terms of moving air, the term will be understood to include dry air, as well as air containing water vapor, and steam.
It is therefore an advantage of the present invention to provide an effective and efficient system and apparatus for removing ammonia compounds and other contaminants from fly ash.